Although the couple married in a secret ceremony in April, the 'Bootylicious' singer had so far refused to show off the band until she was seen on a Miami beach with younger sister Solange last weekend.

The eye-catching piece is believed to be from celebrity jewellers Schwartz, a long-term favourite of the star.

Mohammed Cimark, who runs Precious Gems and Jewels in London, confirmed the huge octagon-cut diamond was a rarity.

He said: "If it is 18 carats then it's a lovely size, an exceptional diamond. It's clearly a premium stone.

"You can't recognise the colour from the photo and the clarity is hard to see, but the best clarity is internally flawless."

The jeweller said he believed the stone had been sourced from Africa.

He added to the MailOnline website: "Most diamonds this size you would get in South Africa or Sierra Leone. That's where you get the biggest diamonds."

The £2.5 million price tag is not likely to have troubled the rapper.

Jay-Z - real name Shawn Carter - reportedly earned £41 million last year from his live performances and album sales, as well as his many other ventures, including a hotel and clothing range.